Ethylene oxide production

ABSTRACT

An improved method is disclosed for the vapor phase epoxidation of ethylene to ethylene oxide which includes intimately contacting an ethylene-containing feed stream with an epoxidizing amount of molecular oxgyen epoxidizing agent in the presence of a catalytically effective amount of a novel supported, activated silver catalyst and, preferably in the presence of an effective amount of at least one inhibitor which retards combustion of ethylene to carbon dioxide at a temperature of from about 200° C. to about 300° C. 
     The novel silver catalyst is best described in terms of its method of preparation. The catalyst is prepared by impregnating certain inorganic porous substrates with a specific silver carboxylate/amine complex impregnating solution. The impregnated support it then heated in order to evaporate volatiles, decompose the complex and activate the catalyst.

This is a division, of application Ser. No. 930,961, filed Aug. 4, 1978,now U.S. Pat. No. 4,206,128, which in turn is a continuation-in-part ofSer. No. 697,088, filed June 16, 1976, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to vapor phase epoxidation reactions of ethylenewith molecular oxygen; and, more particularly to epoxidation reactionsusing specific activated silver catalyst.

2. Description of Prior Art

The preparation of ethylene oxide by the oxidation of ethylene in thepresence of suitable catalysts is well known. These known processes canbe generally separated into two groups; the first utilizes air and thesecond utilizes molecular oxygen, e.g., from about 85 mol percent toabout 99 mol percent. "Silver catalysts" are utilized with both groups.

Although the first references to the use of silver as such a catalystwas made by Walter in British Pat. No. 21,941 (1905), it was not untilsome thirty years later that the first disclosures were made of the useof silver as a catalyst in the vapor phase oxidation of ethylene toethylene oxide. See Societe Francaise de Catalyse Generalisee, FrenchPat. No. 729,952 (1932); and Lefort, U.S. Pat. No. 1,998,878 (1935).

Since silver is expensive, optimizing the amount of silver employed in asupported catalyst for a desired conversion and selectivity to productshas been widely investigated. A variety of techniques have beendeveloped fro the depositing of relatively small, but highly activeamounts of silver on surfaces of non-silver supports such as alumina.For example, McKim and Cambron in Canadian Journal of Research, Volume27, Section B (1949), pp. 813-827, describe a method for depositingparticulate silver on a support by decomposing silver oxalate in aqueousethanolamine at 60° C. and forming a paste which is applied to thesurface of the support. In U.S. Pat. No. 3,043,854 issued July 10, 1962,to Endler, a silver coating formed by decomposition of a silvercarbonate slurry is applied to a catalyst support surface.

Recently it has been disclosed that supported silver catalysts can beprepared by impregnating a porous substrate with certain silvercontaining solutions and evaporating or decomposing the solutions todeposit silver on the substrate. U.S. Pat. No. 3,702,259 to Nielsendescribes the use of an aqueous silver oxalate impregnating solutionwhich employs a solubilizing/reducing agent of ethylenediamine, amixture of ethylenediamine or ethanolamine and ammonia, or a mixture ofethylenediamine and ethanolamine. Van Bylandtlaan, in Belgium Pat. No.808,278 (1974) employs an aqueous solution of hexamethylenetetraminewith an ethylenediamine silver complex to deposit silver on an aluminasupport by decomposition. Additionally, it has been disclosed inJapanese Pat. No. 71/19,606 to Fujii et al that impregnation ofinorganic supports with aqueous silver nitrate/alkanolamine complexeswith subsequent thermal decomposition gives supported silver catalystsfor ethylene epoxidation.

It has now been discovered that an extremely stable, physically durable,supported silver catalyst used in accordance with commercially knownprocesses such as described in U.S. Pat. No. 3,119,837, British Pat.Nos. 1,314,613 and 1,132,095, results in a process which gives higherunit productivity and selectivity to the desired product at loweroperating temperatures. The selectivity is particularly important in airprocesses which are not closed systems because some proportion of theunreacted ethylene is lost by venting excess gas. The catalyst utilizedin the inventive process can be simply produced by impregnating certaintypes of porous, inorganic substrates with a complex formed bydissolving a silver carboxylate in certains amines and thermallydecomposing the complex to deposit the silver on the substrate andactivate the silver. Additionally, the catalysts used in accordance withthe instant process show high attrition resistance and surprisingly highmechanical strength.

SUMMARY OF THE INVENTION

According to the broad aspect of the invention, ethylene is epoxidizedto ethylene oxide substantially in the vapor phase by intimatelycontacting ethylene with an epoxidizing amount of molecular oxygen inthe presence of a catalytically effective amount of an activated silvercontaining catalyst at temperatures of from about 200° C. to about 300°C. Preferably, the ethylene is contacted with the molecular oxygen inthe presence of an effective amount of at least one inhibitor whichretards combustion of ethylene to carbon dioxide.

The activated silver containing catalyst is prepared by impregnatingcertain porous, inorganic substrates with a silver carboxylate/aminecomplex impregnating solution and heating the impregnated substrate attemperature of from about 50° C. to 300° C. to evaporate volatiles,decompose the complex, and activate the catalyst.

The impregnating solution is formed by dissolving a silver carboxylatein a solubilizing amount of an amine-containing complexing agentselected from:

(a) alicyclic diamines wherein at least one amino moiety is primary orsecondary, but no more than one is primary;

(b) polyamines containing at least three amino moieties wherein at leastone is primary or secondary; or

(c) amino ethers containing at least one ether linkage and at least oneamino moiety which is primary or secondary.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with a preferred embodiment, ethylene is epoxidized toethylene oxide in a continuous process by contacting a feed materialwhich comprises from about 5 mol % to about 7 mol % ethylene, from about5 mol % to about 6 mol % oxygen, from about 6 mol % to about 8 mol %carbon dioxide, trace amounts of a suitable inhibitor and the remaindernitrogen. The feed stream is contacted with a catalytically effectiveamount of a specific activated, supported silver-containing catalyst, invapor phase, at temperatures from about 220° C. to about 260° C. and atpressures in the range of from about 13 atm. to about 20 atm.

As more particularly described hereinafter, the supported silvercatalyst used in the instant invention process is prepared byimpregnating a porous inorganic substrate with a silvercarboxylate/amine complex and heating the impregnated substrate at atemperature of from 50° C. to 300° C. to evaporate volatiles, decomposethe complex and activate the catalyst.

The Oxidation Reaction

The epoxidation of ethylene to ethylene oxide can best be described as acontrolled oxidation. It is important to minimize complete oxidation inaccordance with the instant process while maximizing the selectivity andconversion to the desired epoxidized product.

The process parameters used in accordance with the instant invention aregenerally well known in the art. Particular parameters which can effectthe advantageous results obtained by use of the novel activated, silvercatalyst material can be determined without undue experimentation by theskilled artisan in light of the teachings herein.

Generally, the process is carried out in a vapor phase process wherein asingle gaseous feed stream is continuously charged to a suitablecatalyst containing reactor. The catalyst material is contained on asuitable support as hereinafter described.

The reaction is carried out at temperatures from about 200° C. to about300° C., and preferably in the range of about 220° C. to about 260° C.The pressures are not critical and may vary from about atmospheric toabout 35 atm. with about 13 atm. to about 20 atm. being preferred at thepreferred temperature range. The feed admixture is preferably fed in asingle stream to the reactor in order that the constituents bethoroughly admixed.

The feed admixture contains ethylene, molecular oxygen, diluents andinhibitors. The molecular oxygen employed as an epoxidizing agent in theprocess can consist essentially of relatively pure oxygen or thatmolecular oxygen contained in air. A typical concentrated oxygen gas,suitable for use as a make-up oxygen reactant in the process of thisinvention comprises the concentrated oxygen gas consisting of oxygen,nitrogen and argon from the fractional distillation of air. Themolecular oxygen content in this molecular oxygen mixture can range from85 mol % to higher than 98 mol %, with a preferable mol % being 90 to97%. The feed admixture contains from about 3 mol % to about 10 mol %oxygen based on the total feed with 5 mol % to about 8 mol % beingpreferable.

The ratio of oxygen to ethylene in the feed must be of such a value thatacceptable conversion is obtained without creating an explosive mixture.It will be realized by the skilled artisan that the exact mixture willdepend upon such factors as the amount of inhibitor present, the actualinhibitor used, the amount of diluent present, the diluent or mixturesthereof used, as well as the particular temperature and pressureparameters involved in the reaction.

Although dependent on numerous parameters, generally, the ratio ofethylene to molecular oxygen introduced into the ethylene oxidationsystem should be controlled such that the molar ratio of ethylene tooxygen in the feed is from about 0.5 to about 10.0 and preferably in therange from about 1.0 to about 3.0.

Generally, the ethylene can be present in amounts from about 2 to about30 mol % and preferably from about 5 to about 20 mol % based on thetotal feed composition.

The concentration of the feed constituents may be varied by controlledaddition of diluents and/or inhibitors. Diluents which may be present inthe feed can be generally described as those gases which aresubstantially inert to the desired oxidation reaction.

One class of useful diluents includes saturated polyhydrocarbons such asfor example ethane, propane and higher saturated hydrocarbons. It willbe realized by those skilled in the art that saturated hydrocarbonshaving generally more than about 3 carbon atoms are not practical foruse in accordance with the preferred embodiment of the instantinvention. The saturated polyhydrocarbon while being substantially inertto the oxidation reaction, have been shown to effectively promote thecatalytic activity.

Carbon dioxide may also be useful as a diluent. The presence of carbondioxide, however, serves to retard the catalytic activity and thus maylikewise serve as an inhibitor. Gases which are totally inert to thereaction can also be utilized. Examples include methane, nitrogen, argonand the like.

The feed may also contain an inhibitor to minimize complete oxidation ofthe olefin. Any one of a number of commercially available inhibitors canbe employed, such as ethylene dichloride, vinyl chloride,dichlorobenzene, monochlorobenzene, dichloromethane, or chlorinatedbi-phenyls and chlorinated polyphenyls with ethylene dichloride beingpreferable.

The amount of inhibitor utilized can be readily determined by knownmethods. Generally, it can be added in amounts in the range of about0.01 ppm to about 0.5 ppm for a 7-10% molar ethylene composition feed,with 0.01 to 0.05 ppm being preferable. Amounts up to 3.0 ppm may beneeded with ethylene feeds which contain substantial amounts ofpolyhydrocarbons.

The amount of diluent present can generally be determined by the skilledartisan. When methane is present, amounts up to about 30 mol % of thetotal reactor feed may be utilized provided that the totalhydrocarbon-oxygen ratio is kept outside the explosive limits.Polyhydrocarbons such as ethane, may be present in amounts up to about30% if sufficient amounts of reaction inhibitors, such as carbon dioxideor ethylene dichloride are present to balance the reaction-promotingeffect.

Carbon dioxide can be used in concentrations from trace amounts up toabout 50 mol % of the total feed. Amounts from about 2 mol % to 20 mol %are preferred. Preferably carbon dioxide and nitrogen are used as inertdiluents for economic and safety reasons.

The flow rates will vary depending on the feed admixture. These ratescan be readily determined by the artisan. Generally, mass hourly spacevelocities (MHSV) of from about 2.5 grams of feed per gram catalyst perhour to about 25 grams of feed per gram catalyst per hour has been foundsufficient. An MHSV of 5.0 to 8.0 is preferred.

According to a particularly preferred embodiment a single admixed feedstream comprises from 5 to 7 mol % ethylene; from 5 to 6 % oxygen; 6 to8 mol % carbon dioxide, 79 to 84 mol % nitrogen; and trace amounts ofethylene dichloride. The stream is continuously fed to one end of avertical, parallel-tube reactor which is uniformly packed with catalystmaterial which is spherical in shape. The ethylene oxide contained inthe reactor effluent is removed by conventional means such as scrubbersand the like. Gaseous by-products such as carbon dioxide are alsoremoved and the remaining effluent recycled.

The Catalyst

The silver containing catalyst can best be described in terms of itspreparation. A porous, inorganic substrate is impregnated with a silvercarboxylate/amine complex impregnating solution and heated attemperatures of from 50° C. to 300° C. to evaporate volatiles, decomposethe complex, and activate the catalyst.

In accordance with a preferred embodiment, the novel supported silvercatalyst of the instant invention is prepared in four steps. In a firststep, a silver carboxylate/amine complex, as more fully describedhereinafter, is prepared by dissolving a silver carboxylate in an excessof a polyalkylene polyamine having terminal primary nitrogen moieties attemperatures sufficient to dissolve the silver carboxylate.

In a second step, an inorganic porous support, as more fully describedhereinafter, and preferably a high-purity α-alumina support, isimpregnated by immersing the support in the silver carboxylate/aminecomplex at about atmospheric pressure and then subjecting the immersedsupport to vacuum at temperatures of from about 0° C. to about 50° C.and more preferably 20° C. to 40° C. After the vacuum is broken, theexcess complex is drained.

In a third step, the drained support is heated to evaporate volatiles attemperatures of from about 50° C. to 150° C. in a forced-air heater fora time from about 1 to about 12 hours. In a final step, the dried,impregnated support is heated in the presence of forced air attemperatures of from about 150° C. to about 300° C. to decompose thesilver carboxylate/amine complex and activate the supported silvercatalyst material.

The impregnating solution of the instant invention comprises a silvercarboxylate/amine complex which may contain a suitable solvent. Theimpregnating solution can best be characterized as a homogeneous liquidat impregnating temperatures which is formed by solubilizing a silversalt of an organic acid in a solubilizing amount of certainamine-containing complexing agents. Surprisingly, these silvercarboxylate/amine complexes are stable in high silver concentrations atimpregnating temperatures and contain large amounts of silver which arecarried to the support, while simultaneously yielding a solution of aviscosity which is suitable for impregnation of porous, inorganicsupports.

The silver carboxylate/amine complex impregnating solutions of theinstant invention can best be described in terms of their method ofpreparation. Specifically, a silver salt of an organic acid is dissolvedin a solubilizing amount of certain amine-containing complexing agentsat temperatures in the range of from about 0° to about 50° C.

The useful silver salts of organic acids can be generally described assilver carboxylates which readily thermally decompose. Such compoundscan be carboxylates of mono-carboxylic or poly-carboxylic acids.Preferably, the silver salt is of a mono-carboxylic or di-carboxylicacid, wherein the organic moiety contains less than about 10 carbonatoms. Those carboxylates of less than about 20 carbon atoms arepreferred in order to obtain a favorable concentration of silver in theorganic acid salt, and ultimately thus in the complex solution, whileproviding for facile thermal decomposition. It should be noted thatwhile silver salts of organic acids containing more than about 10 carbonatoms are useful, they produce a silver amine complex which becomesincreasingly difficult to decompose as the molecular weight increase andwill reduce the amount of silver ultimately deposited on the support.

Examples of suitable silver carboxylates include silver carbonate,silver acetate, silver malonate, silver glycolate, silver oxalate,silver formate, silver citrate, silver lactate, silver pyruvate, and thelike. The most preferred silver carboxylates are silver oxalate andsilver acetate because of availability and solubility.

The useful amine containing complexing agents of the instant inventioncan be generically described as:

(a) alicyclic diamines wherein at least one amino moiety is primary orsecondary provided no more than one amino moiety is primary;

(b) polyamines containing at least three amino moieties wherein at leastone is primary or secondary; and

(c) amino ethers containing at least one ether (oxy) linkage wherein atleast one amino moiety is primary or secondary.

Although all alicyclic diamines meeting the above criteria are useful ascomplexing agents, a preferred group of such diamines comprisespiperazines, the N-alkyl substituted piperazines and the C alkylsubstituted piperazines.

While all aliphatic polyamines containing at least three amino moietieswherein at least one is primary are useful as complexing agents, apreferred group is the polyalkylene polyamines of the formula ##STR1##wherein R is a straight or branched chain alkylene radical having from 2to about 4 carbon atoms, R₁, R₂ R₃, R₄ and R₅ are, independently,hydrogen or an alkyl radical of from 1 to 5 carbon atoms provided atleast one of R₁, R₂, R₃, R₄ or R₅ is hydrogen; or R₁ and R₂ or R₃ and R₄with the nitrogen to which they are attached form a piperazine ring andn is an integer of from 1 to about 4. Examples includeN(aminoethyl)-piperazine, N,N'-bis(2-aminoethyl)piperazine,diethylenetriamine, N-methyldiethylenetriamine, triethylenetetramine andthe like. The most preferred polyalkylene polyamine compounds arediethylenetriamine and triethylenetetramine.

The amino ethers that are useful within the scope of the instantinvention are the saturated and unsaturated, substituted andunsubstituted aliphatic amino ethers. These compounds may be straight orbranched chain, acryclic, alicyclic, heterocyclic or cyclic. Examples ofsuch compounds include morpholine, the C-substituted morpholines, etc.;the bis(aminoalkyl) ethers, the n-alkyl bis (aminoalkyl) ethers, etc.;the polyoxyalkylene amines; the polyoxyalkylene polyamines, etc.; thealkoxyalkyl amines; amino-containing ethers derived from furan; and thelike.

One preferred class of amino ethers is morpholine and the C-alkylsubstituted morpholines. Another preferred class is thepolyoxyalkyleneamines of molecular weight less than 1,000 and morepreferably less than 500. Examples include the polyoxypropylenediaminesof molecular weight less than about 400, and a polyoxypropylenetriamineof molecular weigt about 500. Both of the above types of compoundscontain terminal primary amino groups.

The amount of a particular amine containing complexing agent utilized informing the silver caroxylate/amine complex inpregnating solution issomewhat empirical. Generally that amount of amino complexing agentsufficient to completely dissolve the required amount of silver salt,which can be determined by observation. The amount of silver saltrequired is somewhat empirical and generally determined by the amount ofsilver ion required in solution and the porosity of the support.

As hereinbefore mentioned, it is desirable to have the complex as "rich"as possible in silver. Generally, the impregnating solution shouldcontain an amount of about twice that desired in the finished catalyston a wt. % basis with a support having about a 50% porosity. It ispreferable, therefore, to obtain a complex which contains more thanabout 10 wt. % silver in the impregnating solution and more preferablyfrom about 12 to about 25 wt. % silver.

When the preferred polyalkylene polyamines are utilized, it is desirableto have from about 1 to about 6 amine equivalents of silver in order toform the optimum complex containing solution.

The silver salt is preferably solubilized in the amine containing agentat temperatures in the range of about 20° C. to about 40° C.Temperatures in excess of 50° C. are not preferred, since highertemperatures tend to cause accelerated decomposition of the complex.

If desired, solubilizers can be added in order to facilitate dissolutionof the silver salt in the amine complexing agent. Examples of suchsolubilizers include water, aqueous ammonia, and the like. In accordancewith a preferred embodiment, water is utilized as the solubilizingagent. Water only reduces the viscosity of the impregnating solution,reduces the amount of amine required to solubilize the silver salt, andreduces potential hazards of handling the solution, but also acts as asolvent for the silver salt/amine complex, thus preventing prematureprecipitation.

Examples of suitable solubilizers include aqueous methylamine,ethylamine, diethylamine, triethylamine, and pyridine. It is, however,recognized that the marginal advantages of such solubilizers may beoutweighed by the fact that certain lower molecular weight amines orammonia can form explosive solids with silver. In addition, although notnecessary, small amount of hydrogen peroxide or other suitable oxidizingagents may be added to minimize premature reduction of the silver in thecomplex.

The Support

The support utilized to form the novel silver catalyst of the instantinvention can be generally described as a porous, inorganic substratehaving those characteristics which are well known in the art andparticularly known in the ethylene epoxidation art. Suitable supportswhich can be used in accordance with the instant invention are glass,alumina, silica, silica-alumina, inert metals, silicon carbide andzirconia. It is essential that the support chosen have a high porosity(i.e., high solvent absorption), low surface area and a controlled portsize. Preferably, from about 70% to 100% of the pore diameters arebetween about 1 and 30μ and more preferably between about 1 and about10μ.

A preferred support media has an average pore diameter of from about 4to about 6μ0 with a pore volume of from about 0.3 to about 0.6 cc/g andhas a surface area less than about 1 m² /g. A particularly preferredsupport is high purity α-alumina having the above characteristics.

Preparation of the Supported Silver Catalyst

In preparing the stable, supported silver catalyst of the instantinvention, a suitable support is first contacted with the impregnatingsolution or mixtures thereof and subsequently heated at elevatedtemperatures to first evaporate the volatiles and finally to decomposethe silver carboxylate/amine complex and activate the catalyst material.Although the preparation of the supported catalyst can be accomplishedin two steps; i.e., an immersion step and an evaporation, activating,and decomposition step at incrementally increasingly elevatedtemperatures, it is preferable to prepare the catalyst of the instantinvention in three distinct steps.

After the impregnating solution has been prepared, as describedhereinabove, the substrate to be impregnated is contacted with thesolution in a first step. This is preferably accomplished by immersionof the substrate in a suitably large body of impregnating solution tocompletely cover the substrate. The immersed substrate is then subjectedto an evacuated atmosphere for a time period sufficient to removeentrapped air from the support pores at temperatures of from about 0° C.to about 50° C. and more preferably from about 20° C. to about 40° C.

The impregnation time will depend on the characteristics of thesubstrate and the viscosity of the impregnating solution and can bereadily determined by the skilled artisan. Although somewhat empirical,it is generally sufficient to contact the porous substrate with theimpregnating solution for a time from about five minutes to severalhours. When utilizing impregnating solutions of silver salts ofpolyalkylene polyamines, a time from about ten minutes to two hours issufficient. After the substrate has been contacted for sufficient timeunder vacuum, the vacuum is broken to return the pressure to atmosphereand then the excess solution is physically drained from the substrate.

In a second step the drained substrate is dried in the presence ofheated flowing air, or a heated flowing inert atmosphere, attemperatures from about 50° C. to 150° C. for a period sufficient toevaporate the volatiles. Generally the time required to dry theimpregnated substrate is somewhat empirical and can be readilydetermined by the skilled artisan for a particular substrate andimpregnating solution. Time periods of from about one to about twelvehours have been found sufficent. It should be noted that during thedrying step temperatures in excess of about 150° C. should be avoided asthe complex may tend to decompose too rapidly and/or cause the volatilesto evaporate so readily as to disturb the uniformity of the catalystmaterial. Although not required, it is found that first thoroughlydrying the impregnated substrate prior to thermal decomposition yields amore uniform catalyst.

In the third step the dried impregnated substrate is heated in thepresence of flowing air, or a flowing inert atmosphere to temperaturesin excess of about 180° C. and preferably from about 200° C. to about300° C. to decompose the complexing agent and activate the supportedsilver catalyst materials. The time required to thoroughly decompose thesilver salt/amine complex and activate the catalyst is somewhatempirical but generally times in the range from about one to abouttwelve hours have been found sufficient.

It will be realized by the skilled artisan that when other solubilizingagents such as water, aqueous ammonia, aqueous alkylamines, and the likeare present in the complexing agent in accordance with the instantinvention the times required for drying may be somewat variabledepending on the solubilizing agents. The specific times required aregenerally within the above broad limits and can be determined by theskilled artisan without undue experimentation. Additionally, when highermolecular weight amines are utilized, washing of the dried substrate maybe advantageous to remove excess organic material prior to activation.The washing may be accomplished in a conventional manner with loweralkanols or other suitable solvents.

The instant invention will be further illustrated by the followingspecific example, which are given by way of illustration and not aslimitations on the scope of this invention.

EXAMPLE I

This example illustrates preparation of the stable supported silvercatalyst employed in the instant invention. In a first step, silveroxalate was prepared. To an appropriate clean, dry vessel equipped withstirring apparatus were charged a solution of 18.4 g potassium oxalatedissolved in 150 cc of deionized water and a solution of 34.0 g silvernitrate in 150 cc deionized water. The two solutions were admixed at 60°C. and atmospheric pressure by stirring for several minutes. The mixturewas then filtered and the residue washed with four aliquots of hot,deionized water totaling 50 cc. The residue was then further washed withtwo 25 cc aliquots of absolute methanol. The residue was then partiallyair dried by evacuating the lower portion of the filter surface.

In a second step, the dried silver oxalate and 30 ml of deionized waterwere added to a clean dry breaker and stirred until a slurry wasobtained. To the stirred slurry was added 10 cc of NH₄ OH (30% NH₃ byweight) and 25 g of diethylenetriamine (DETA) yielding a dark, opaguehomogeneous solution having a mol ratio of Ag/DETA of about 0.65.

In a third step the solution prepared in step two was used as animpregnating solution. The impregnating solution was drawn into anevacuated clean dry 150 ml stainless steel sampling cylinder containing75 g of a commercial pure alumina support (3/16" spherical pellets)having a pore volume of 0.41 cc/g, a surface area of less than about 1m² /g and an average pore diameter of 5.9μ. The vacuum was maintainedfor about 10 to 15 minutes until the pressure had dropped to about 10 mmHg. The vacuum was then released, and the container pressurized to 200psig with nitrogen. After warming the cylinder to ambient temperature,the contents were allowed to stand under pressure for 30 minutes. Thepressure was then released and excess solution drained.

In a fourth step the cylnder containing the wet impregnated material wasattached to a forced-air heater, and heated to approximately 130° C. for1 hour to dry the wet material. The dried material was then allowed tocool overnight.

In a fifth step the cylinder and contents were reheated to about 130°C., and then the temperature was raised to 250° C. over a period ofabout 1 hour and held at that temperature for an additional hour. Aftercooling, the recovered material weighed 85 g and had a silvery-tanappearance. Upon inspection, the interior of the supported catalystappeared somewhat non-uniform and the material was again heated at 250°C. for an additional hour. After cooling, it was determined by analysisthat the material contained 12.2 weight percent silver.

EXAMPLE II

This example illustrates the preparation of a supported catalystemployed in the process of the instant invention using a silveroxalate/diethylenetriamine impregnating solution. Silver oxalate wasinitially prepared, as described in the first step of Example I, addedto 30 cc deionized water and stripped to form a slurry. To the slurrywas added a mixture of 30 g diethylenetriamine and 10 cc deionizedwater, forming a dark, opague impregnating solution. The preparedimpregnating solution was used to immerse 50 g of the pure α-aluminasupport material, as described in Example I (1/4" spherical pellet). Thesupport and covering solution was then placed under full pump vacuum.The vacuum was released to atmospheric pressure and the above sequencewas once repeated. Upon draining, the wet impregnated support materialwas placed into a 150 cc sampling cylinder, which was attached to aforced-air heater, and dried at a temperature of about 120° C. for anhour. The sampling cylinder containing the dried material was thenheated to about 250° C. over a 30-minute period and held at thattemperature for an hour. After cooling, the recovered impregnatedmaterial weighed 56 g and had a silvery-gray appearance. Analysis showedpresence of 11.0 weight percent silver.

EXAMPLE III

This example illustrates catalysts prepared for the process in theinvention using an impregnating solution of silver oxalate andbis(2-aminoethyl)ether (BAEE). Silver oxalate was prepared as describedin the first step of Example I and added to 50 cc deionized watercontaining 10 cc of NH₄ OH (30% NH₃ by weight) solution to form astirred slurry. To the slurry was slowly added 25 g of distilled BAEE.An additional 10 cc of the NH₄ OH (30% NH₃ by weight) solution was addedto dissolve the remaining undissolved materials. A dark homogeneoussolution resulted.

Following the procedure in step 3 of Example I, the solution was drawnby suction into an evacuated 150 cc stainless steel sampling cylindercontaining 75 g of the catalyst material described in Example I. Thesupport was impregnated and dried substantially as described in ExampleI except that the drying temperature was 120° C. The dried catalyst wasthen treated in accordance with the procedure of step 5, Example I toproduce 80 g of a uniform gray-tan material which, upon analysis, wasshown to contain 7.7% by weight silver.

EXAMPLE IV

This example illustrates preparation of a catalyst employed in theinvention using a silver oxalate/polyoxyalkyleneamine compleximpregnating solution. As in Example I, the silver oxalate was preparedand added to 30 g deionized water to form a stirred slurry. To theslurry was added 60 g of a polyoxypropylenediamine (nw 230) sold underthe tradename JEFFAMINE® D-230 by Jefferson Chemical Company, Inc. About5 cc of aqueous NH₄ OH (30% NH₃ by weight) solution was added tosolubilize the remaining trace amount of solids. The solution was drawnby suction into a cylinder which contained support material, asdescribed in Example I. After impregnation, the remaining solution wasdrained. Drying was accomplished with forced air at a temperature of120° C. for three hours. Prior to decomposition, the dried material waswashed with anhydrous methanol for five hours in an extraction thimbleto extract undecomposed organic matter. Although this washing step isnot necessary, it may be utilized to facilitate removal of highmolecular weight organic matter which may form a residue during thedecomposition step at higher temperatures. In this example, the methanolwet catalyst was returned to the sampling cylinder and again dried at120° C. for one hour prior to being heated at 250° C. for an additionalhour to effect decomposition and activation. Upon cooling, the recoveredmaterial weighed 80 g, being slightly gray in color. Upon analysis, thematerial was shown to contain 9.0 weight percent silver.

EXAMPLE V

This example illustrates preparation of a catalyst used in the inventionfrom silver oxalate and iminobis(propylamine) (IBPA) impregnatingsolution. Silver oxalate was prepared as described in Example I andadded to a beaker containing 30 g of deionized water. After ahomogeneous slurry was formed, 10 ml of concentrated NH₄ OH (30% NH₃ byweight) was added; followed by 30 g of iminobis(propylamine). Thesolution was used, as in Example I, to impregnate 50 g of the 3/16"spherical alumina support of Example I. When the solution was drained,it was observed that crystallization of solids from the impregnatingsolution had occurred. The semi-solid was separated from the impregnatedsupport using pressurized air. The contents of the cylinder were driedfor about 17 hours with a forced-air heater at approximately 230° C. Thedried material was then heated to about 250° C. over a period of onehour. The temperature was maintained for an additional hour. Aftercooling, the finished catalyst had a silvery-gray color and was observedto have a fairly heavy silver coating on the external surface. Thecatalyst, upon analysis was shown to contain 16.8 weight percent silver.

EXAMPLE VI

In this example, a prior art catalyst using silver oxalate,ethylenediamine and monoethanolamine was prepared in accordance with theprocedure of Example I. Silver oxalate was prepared, as described inExample I, and added to a beaker containing 50 cc deionized water. Tothe resulting slurry was added a mixture of 14 g ethylenediamine (EDA)and 14 g monoethanolamine (MEA). The resulting solution was drawn bysuction into an evacuated 150 cc stainless steel sampling cylindercontaining 75 g of the 3/16" spherical support of Example I and theimpregnation was carried out as therein described. The wet support wasdried in a forced air heater at approximately 130° C. for two hours, andheated at approximately 250° C. for 3 hours in dry air. The material wasa light tan color and contained 10 weight percent silver.

EXAMPLE VII

The six catalysts described above in Examples I-VI were tested for theprocess disclosed in a miniature ethylene oxide reactor, a 0.2 inch I.D.stainless steel tube, ten inches in length, operating at 200 psig andusing 3.5 g of 30-40 mesh catalyst per test. The feed composition wasapproximately 7 mol % ethylene, 6 mol % oxygen, with the balancenitrogen and trace amounts of ethylene dichloride inhibitor. The reactorwas operated at a temperature of 250° C. and mass velocity of about 5 gof feed per gram of catalyst per hour. Enough moderator was added togive maximum selectivity at the chosen ethylene oxide production rate.Selectivities and conversions are given in mol percent. Results aregiven in Table I.

                  TABLE I                                                         ______________________________________                                        Catalyst prepared according to indicated Examples                             I           II      III     IV    V      VI.sup.2                             (12.2).sup.1                                                                              (11.0).sup.1                                                                          (7.7).sup.1                                                                           (9.0).sup.1                                                                         (16.8).sup.1                                                                         (10.0).sup.1                         ______________________________________                                        Selectivity                                                                           73      70      72    71    73     70                                 to ethy-                                                                      lene oxide                                                                    C.sub.2 H.sub.4                                                                       34      41      34    36    32     35                                 Normalized                                                                            73      72      72    72    73     70                                 selectivity                                                                   at constant                                                                   conversion                                                                    of 32%                                                                        Reactor 250° C.                                                                        250° C.                                                                        250° C.                                                                      250° C.                                                                      230° C..sup.3                                                                 250° C.                     temper-                                                                       ature                                                                         ______________________________________                                         .sup.1 Wt. % Ag by analysis .sup.2 Prior art catalyst using                   solubilizing/reducing impregnating solution in accordance with the            procedures of the instant invention                                           .sup.3 A very high silver content made it impractical to operate this         active catalyst at 250° C.                                        

EXAMPLE VIII

Large scale preparation of catalyst for the invention using silveroxalate and DETA.

A large batch of fresh silver oxalate was prepared as follows: A 60° C.solution containing 102 g of silver nitrate, A.R., and 500 cc ofdeionized water was added slowly with stirring to an approximately 60°C. solution of 44 g of ammonium oxalate slurry, A.R., in 500 ccdeionized water. The silver oxalate slurry was stirred for 20 minutes,then filtered through a Buchner funnel, washed with 300 cc deionizedwater and then with 300 cc anhydrous methanol in small portions. Thesilver oxalate was partially dried under aspirator vacuum of 10 mmmercury and added slowly to a beaker containing 100 cc deionized waterto form an aqueous slurry. The slurry was chilled to below roomtemperature with an ice bath while a solution containing 90 g DETA and30 ml deionized water was added slowly, keeping the solution temperaturebelow 60° C. The silver solution was removed from the ice bath after allthe DETA solution had been added and was stirred until all solids haddissolved.

The impregnating solution thus formed was added in sufficient quantityto cover the surface of 326 g of the 3/16" spherical alumina support (asdescribed in Example I). The support and solution were placed under fullpump vacuum, the vacuum released to atmospheric pressure and theimmersed catalyst allowed to stand undisturbed for 30 minutes. The wetsupport was drained in a wire basket, then charged to a 500 cc stainlesssteel sampling cylinder which was attached to a forced-air heater. Theimpregnated support was dried at approximately 125° C. for one hour,then heated to 250° C. over a period of one hour, and maintained at 250°C. for one hour. After cooling, the catalyst was a uniform gray colorinside and outside, weighed 259 g, and contained 9.7 weight percentsilver.

The catalyst was tested in a pilot plant reactor operated at 245° C.with a total gas feed rate of about 896 liters/hour. As before, the feedconsisted of 7% ethylene, 6% oxygen, but now contained 7.5 to 8% carbondioxide, in addition to the other components.

Upon analysis, there was shown: conversion of ethylene, 25%; selectivityto ethylene oxide, 75% and ethylene oxide in effluent gas, 1.23 mol %.The productivity of the catalyst was 0.127 g ethylene oxide per gram ofcatalyst per hour.

A second test was then run using a prior art silver catalyst which isused in a commercial ethylene oxide process under substantiallyidentical conditions. The commercial catalyst showed a maximumproductivity of 0.0977. Thus, the catalyst of the instant inventionshows an increase in productivity of about 20-25%.

EXAMPLE IX

The following example demonstrates the superior complexingcharacteristics of the amine containing complexing agents used in theinstant invention. Four separate impregnating solutions were preparedsubstantially as in Example I, keeping the solution at a temperaturebelow 50° C. Silver oxalate was used as the silver salt. The formedsolutions and their respective characteristics are listed in Table II.

                  TABLE II                                                        ______________________________________                                        COMPLEXING AGENT                                                                                      N-ethyl-  Tetrahydro-                                 Morpholine    Piperidine                                                                              morpholine                                                                              furfuryl                                    (50 ml)       (50 ml)   (50 ml)   amine (50 ml)                               ______________________________________                                        initial homo-     silver    slight or                                                                             homogeneous,                              solubility                                                                            geneous,  plated    none,   stable                                    of silver                                                                             stable    from      two phase.sup.3                                   oxalate.sup.1     solution                                                    appearance                                                                            trace of  large     same as Trace of                                  on standing                                                                           finely di-                                                                              amount    above, no                                                                             finely divided                                    vided black                                                                             of crystal-                                                                             change  precipitate                                       precipitate                                                                             line                                                                          material                                                    catalyst                                                                              no diffi- N/A.sup.2 N/A.sup.2                                                                             No difficulty                             impreg- culty                                                                 nation                                                                        ______________________________________                                         .sup.1 Silver oxalate slurried in 30 ml water.                                .sup.2 No catalyst preparation  solution unsatisfactory for attempt.          .sup.3 A second aliquot of 50 ml Nethyl morpholine was added with no          change.                                                                  

While the invention has been explained in relation to its preferredembodiment, it is to be understood that various modifications thereofwill become apparent to those skilled in the art upon reading thespecification and is intended to cover such modifications as fall withinthe scope of the appended claims.

What is claimed is:
 1. An improved method for epoxidizing ethylene toethylene oxide in the vapor phase, which comprises the stepof:intimately contacting ethylene with an epoxidizing amount of amolecular oxygen epoxidizing agent in the presence of a supported silvercatalyst at epoxidizing temperatures from about 200° C. to about 300°C., wherein the catalyst is prepared by; contacting a porous, inorganiccatalyst support material with an impregnating solution comprising asilver carboxylate/amine complex; and, heating the impregnated supportmaterial at temperatures from about 50° C. to 300° C. to evaporatevolatiles, decompose said complex and activate said catalyst, whereinsaid silver carboxylate/amine complex comprises a silver carboxylatedissolved in a solubilizing amount of an amine containing complexingagent and wherein the said complexing agent is tetrahydrofurfurylamine.2. The method of claim 1 wherein the said ethylene is contacted with amolecular oxygen epoxidizing agent in the presence of the said supportedsilver catalyst and in the presence of an effective amount of at leastone inhibitor to retard the combustion of ethylene to carbon dioxide. 3.The method of claim 1 wherein the said ethylene is contacted with amolecular oxygen epoxidizing agent in the presence of said supportedsilver catalyst and in the presence of an effective amount of ethylenedichloride to retard combustion of ethylene to carbon dioxide.
 4. Themethod of claim 1 wherein the said inpregnating solution furthercontains a substantial amount of water.
 5. The method of claim 1 whereinthe said silver carboxylate is selected from the group consisting ofsilver salts of monocarboxylic acids, dicarboxylic acids and mixturesthereof wherein the organic moiety contains less than about 10 carbonatoms.
 6. The method of claim 1 wherein said silver carboxylate isselected from a group consisting of silver carbonate, silver acetate,silver malonate, silver glycolate, silver oxalate, silver formate,silver citrate, silver lactate and silver pyruvate.
 7. The method ofclaim 1 wherein said support material is a high purity α-aluminamaterial having an average pore diameter of from about 4 to about 6μwith a pore volume of from about 0.3 to about 0.6 cc/g and a surfacearea less than about 1 m² /g.
 8. The method of claim 1 wherein theepoxidizing temperature is between about 220° C. and 260° C., and apressure of from about 13 atmospheres to 20 atmospheres.